The present invention is generally directed to an apparatus and a method for manufacturing articles of polyurethane foam. More particularly, the invention relates to an apparatus and a method for manufacturing relatively large polyurethane articles by varying the pressure in the mold in which the article is formed.
Polyurethane foam has been used for many years for cushioning, insulation and other applications. Polyurethane foam is usually manufactured at atmospheric pressure from polyester or polyether based polyols combined with isocyanates, such as toluenediisocyanate (TDI), polymethylene polyphenylisocyanate (MDI), or mixtures thereof, and additives to form a finished product ranging from a very flexible to a very rigid product. The cell structure of the foam can range from completely open to completely closed. Examples of open cell, flexible polyurethane foam technology are disclosed in U.S. Pat. No. 4,451,583.
Most expanded flexible polyurethane is currently produced by the continuous conveyor method for producing slabs or by molding articles in a noncontinuous process. The continuous conveyer method or "slabstock" method is used to process the majority, by weight, of flexible polyurethane foam in the world. In this method, the liquid chemicals are mixed together and poured on a carrier sheet of plastic or paper. The carrier sheet rests either on a conveyor flat floor with two vertical sides or on a conveyor with a round shape. As the chemicals proceed down the conveyor, they rise or expand in the form of closed cells. In the case of open cell foam, as the reacting chemicals reach full expansion, the cell walls open and flow into struts. These struts continue to solidify until an almost cured dry article is formed. At the end of the conveyor, a saw cuts off a length of the article. The article is then taken to a storage area for final curing, which usually takes about 24 hours. This process is continuous until the machine is stopped.
An alternative method for producing polyurethane articles is the molding method. In the molding method, the liquid chemicals are mixed and deposited in a mold, with or without a lid, and the chemicals expand to the shape of the mold. An example of a molding apparatus is disclosed in U.S. Pat. No. 4,404,168.
As is well known to those skilled in the art and explained in U.S. Pat. No. 4,601,864, the above manufacturing methods have the disadvantage of releasing undesirable gases such as isocyanate, chlorofluorocarbons, or chlorinated solvents. It is known that these gases are harmful to either worker health or the environment.
One method to solve the chlorofluorocarbon and chlorinated solvent emission problem is to use a vacuum in place of these chemicals as proposed in European Patent Application, Publication No. 0,023,749. However, to those skilled in the art, it is known that to be economically successful the polyurethane article must be relatively large in size and have substantially flat surfaces and substantially square edges, all of which are difficult to achieve using the method in European Patent Application, Publication No. 0,023,749.
The present invention solves this problem by using vacuum in a manner that produces a relatively large polyurethane article that has substantially flat surfaces and substantially square edges all of which allow for economical fabrication of the article. The present invention also solves the problem of worker health and isocyanate fume emission by enclosing the machine and capturing the isocyanate gas in charcoal filters.